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SMD Measurement Tweezers

While fiddling around with those SMD capacitors, it occurred to me that I really needed some SMD tweezers: small forceps with isolated jaws, connected to the capacitance meter’s terminals. In the nature of a proof-of-concept, I sacrificed a (surplus) Tektronix banana plug cable and an old plain-steel tweezer (stamped Made in Japan back in the day when that had the same quality connotations as does Made in Pakistan right about now) and lashed them together:

SMD tweezers - overview

SMD tweezers - overview

I chopped off the tweezer joint with a bolt cutter, scuffed up the steel with a file, soldered the cable wires, cut a small wood block to fit, and epoxied the whole mess together:

SMD tweezers - epoxy joint

SMD tweezers - epoxy joint

When the epoxy cured, a generous wrap of silicone tape hid most of the hackage. Two lengths of clear heatstink tubing insulate the handles from my sweaty fingers:

SMD tweezers - joint detail

SMD tweezers - joint detail

Part of the reason for picking this victim was its cheap-and-bendy steel: more easily soldered than stainless, no regrets about filing the jaws to suit. They’re flattened on the bottom and filed to grip SMD chips along their length:

SMD tweezers - tip shape

SMD tweezers - tip shape

That’s on the top panel of my indispensable AADE LC meter. The stray capacitance of that cable is around 50 pF, but the meter can null it to a fraction of a pF. At least as long as I don’t change my grip, that is, which isn’t too severe a restriction. [Update: got the link right this time.]

That gorgeous Tek cable turned out to be entirely too stiff and the natural curve doesn’t lie in the correct direction. The next version will probably use a length of RG-174 mini coax and a dual banana plug. I think I’d like angled jaws, too, so as to attack the chips from the top down.

But even this version works wonderfully well, as I sorted out a few hundred random SMD caps in two half-hour sessions that I’d been putting off for far too long. This is the last batch; I’ve learned the hard way that it pays to transfer batches of chips to their storage bins long before I think I should:

Sorting SMD caps

Sorting SMD caps

Yeah, it’s false economy, but it keeps me off the streets at night. OK?

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  1. #1 by smellsofbikes on 2011-12-08 - 13:59

    I was *just* thinking about building one of those, and this is totally inspirational. I have a huge pile of failed Tektronix scope probes, where the failure is in the cable, and was thinking those would make nice supple low-loss cables for a project like this. Drill two holes in the tweezer joint, yank it in twain, put some polyimide tape between them and reattach them with nylon screws, and go from there. Hmmmm.

    • #2 by Ed on 2011-12-08 - 14:44

      polyimide tape between them

      That might give you too much parasitic capacitance; the random chunk of wood I used was about 5 mm thick and worked just fine.

      I’ll grant you that the nylon screw thing seems dramatically less ugly…

    • #3 by John Rehwinkel on 2011-12-09 - 16:04

      A lot of scope probes use special coax with a distributed-resistance center conductor that can be fragile and difficult to solder to.

  2. #4 by david on 2011-12-08 - 19:17

    What, no log/log graph paper? For shame! :)

    (I think I’d’ve soldered a pair of dead-bug PCB punched islands (that was you, right?) to the tip and run the coax all the way down one side and jumpered over with an inch of center conductor. Keep the tweezers out of the circuit -> no need to insulate or deal with grip issues…

    • #5 by Ed on 2011-12-08 - 19:34

      no log/log graph paper?

      Wait ’til you see the resistor sorting sheet… [grin]

      jumpered over with an inch of center conductor.

      Would that introduce a higher ratio of capacitance change as the tips move? It’d certainly have a lower total capacitance, though, so maybe it wouldn’t matter.

      I’ll try that out when I build the next set, which could be a while.

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